The present invention relates to an image forming apparatus such as a printer and a copier, and to a fixing device disposed in the image forming apparatus.
In a conventional fixing device disposed in an image forming apparatus such as a printer, a fixing belt in an endless shape is extended between two opposite pressing rollers, so that the fixing belt rotates and moves while a heating member heats the fixing belt. Accordingly, it is possible to transfer developer such as toner formed of charged fine particles to a printing medium at a high speed with low power consumption. When the printing medium passes through between the pressing rollers and contacts with the fixing belt, developer is fixed to the printing medium.
Patent Reference has disclosed such a conventional fixing device. The conventional fixing device includes a first pressing roller, a second pressing roller, and a plane heating member. The first pressing roller is provided for pressing the printing medium through the fixing belt. The second pressing roller is disposed to face the first pressing roller for contacting with an outer circumferential surface of the fixing belt to form a nip region (a contacting surface where the pressing rollers are pressed against each other), thereby pressing the printing medium. The plane heating member is arranged to contact with the fixing belt at an upstream side of the nip region in a moving direction of the fixing belt for heating the fixing belt.    Patent Reference: Japanese Patent Publication No. 2007-322888
In the conventional fixing device disclosed in Patent Reference, the plane heating member includes an electrode connected to a power source at an end portion of a main body thereof for receiving a voltage. Further, the plane heating member is attached to a supporting member, and has an identical resistivity as a whole. When the power source applies a constant voltage to the plane heating member, the plane heating member is uniformly heated up.
In the conventional fixing device disclosed in Patent Reference, an end portion of the plane heating member tends to dissipate heat to surrounding to a greater extent than a middle portion of the plane heating member. Accordingly, even when an entire body of the plane heating member is uniformly heated up, a temperature at an edge portion of the fixing belt tends to be lower than that at a middle portion of the fixing belt when the plane heating member heats the fixing belt.
In particular, the plane heating member tends to dissipate heat through the supporting member. As a result, the printing medium tends to have a temperature distribution, so that it is difficult to uniformly fix developer to the printing medium, thereby causing a fixing variation spot of developer on the printing medium. When developer is pressed and fixed to the printing medium, if developer is heated insufficiently, developer is not completely melt. Accordingly, a part of developer may stick to the fixing belt, thereby causing phenomenon called cold offset.
In view of the problems described above, an object of the present invention is to provide a fixing device and an image forming apparatus capable of solving the problems of the conventional fixing device. In the present invention, it is possible to prevent cold offset on a side edge of a printing medium, and to uniformly fix developer to the printing medium.
Further objects and advantages of the invention will be apparent from the following description of the invention.